KR20080098025A - Exhaust system comprising zoned oxidation catalyst - Google Patents

Abstract

An exhaust system for a lean-burn internal combustion engine, comprising a catalyst for oxidising carbon monoxide (CO) and hydrocarbons (HC), which catalyst comprising a flow-through substrate monolith (10) comprising a first washcoat zone (14) of substantially uniform length containing at least one platinum group metal (PGM) supported on a surface area-increasing washcoat component, which first washcoat zone being defined at an upstream end by an inlet end (12) of the substrate monolith and at a downstream end (16) by a point less than half way along a length of the substrate monolith measured from the inlet end, a second washcoat zone (18) containing at least one PGM supported on a surface area-increasing washcoat component, which second washcoat zone being of substantially uniform length and comprising the point half way along the substrate monolith length measured from the inlet end and a third washcoat zone (22) containing at least one PGM supported on a surface area-increasing washcoat component of substantially uniform length, which third washcoat zone being defined at a downstream end by an outlet end of the substrate monolith and at an upstream end by a point (20) at most three quarters of the way alo ng the substrate monolith length measured from the inlet end, wherein both the PGM loading in the first washcoat zone and the PGM loading in the third washcoat zone is greater than the PGM loading in the second washcoat zone and wherein the first washcoat zone comprises a washcoat loading that is less than a washcoat loading of the third washcoat zone.

Description

EXHAUST SYSTEM COMPRISING ZONED OXIDATION CATALYST}

The invention is a lean burn internal combustion engine, suitably light-duty, comprising a catalyst for oxidizing carbon monoxide (CO) and hydrocarbons (HC), such as soluble organic fractions (SOF) of particulate matter in diesel exhaust. An exhaust system for a diesel engine.

US 6,827,909 Bl (incorporated herein by reference in its entirety) is first and second coated side by side with the catalytically active material, the passages through which the exhaust gases can flow, the walls separating the passages from each other, and the first and second arranged side by side in the flow direction. Disclosed is a catalytic converter for purifying exhaust gases from a diesel engine comprising at least one honeycomb having a zone. The first zone has a lower heat capacity per unit volume of the honeycomb than the second zone. In one embodiment, the first and second zones are formed of coatings having different thicknesses. The first zone can quickly reach its operating temperature at high exhaust gas temperatures, while the second zone stores heat at operating conditions associated with low exhaust gas temperatures.

WO 01/74476 (incorporated herein by reference in its entirety) discloses a multizone NO _x absorber comprising a first zone, a second zone and optionally at least one intermediate zone disposed between the first and second zones. . In the first zone, the catalyst trap material comprises a palladium catalyst component in an amount of about 30 to about 300 g / ft ^-3 , a platinum catalyst component in an amount of 0 to about 100 g / ft ^-3 , and 0 to about 10 g / ft ^-3 A refractory metal oxide support in which a positive rhodium catalyst component is dispersed; And a NO _x absorbent comprising at least one basic oxidation compound of at least one alkaline earth metal and optionally at least one basic oxidation compound of at least one alkali metal. In the second zone, the catalytic trap material is from 0 to about 50 g / ft ^-3 amount of a palladium catalytic component, about 10 to about 100 g / ft ^-3 amount of a platinum catalytic component, and from about 5 to about 20 g / ft ^- A refractory metal oxide support having ^three rhodium catalyst components dispersed therein; And a NO _x absorbent comprising at least one basic oxidizing compound of at least one metal selected from the group consisting of alkali metals and alkaline earth metals. In any zone (s), the catalyst trap material may contain a palladium catalyst component in an amount of about 25 to about 75 g / ft ⁻³ , a platinum catalyst component in an amount of about 5 to about 30 g / ft ⁻³ , and 0 to about 10 g / a refractory metal oxide support having a ft ^-3 amount of rhodium catalyst component dispersed therein; And a NO _x absorbent comprising at least one basic oxidizing compound of at least one metal selected from the group consisting of alkali metals and alkaline earth metals. Thus, this disclosure suggests that the first zone, followed by the intermediate zone, and then the last second zone, may have a maximum total platinum group metal loading of 410, 115 and 170 gft ⁻³ , respectively.

The European New Fuel Driving Standard (NEDC) is used to identify emissions from lightly loaded diesel vehicles in Europe. The NEDC consists of four ECE sections (which are repeated one after another) followed by one European Urban Driving Cycle (EUDC) section. There is no idling period before starting the first ECE part. The ECE standard is a city fuel consumption standard (also known as UDC) and is intended to represent, for example, a city fuel consumption standard in Paris or Rome. It is characterized by low vehicle speeds, low engine loads and low exhaust gas temperatures. The EUDC section evaluates more aggressive driving and high speed modes of operation. NEDC is also known as the MVEG-A standard. Test standards equivalent to NEDC in the United States are FTP-75 supplemented with the US06 part reflecting aggressive and high-speed operation and the SC03 part for testing emission standard compliance during air-conditioning operation. Other countries have adopted their equivalent test standards.

Catalysts for oxidizing at least carbon monoxide (CO) and hydrocarbons (HC) in exhaust gases emitted from lean combustion engines typically comprise one or more expensive platinum group metal (s) (PGMs) such as platinum, palladium and / or rhodium. Catalyst manufacturers are competitive if they can develop new oxidation catalysts that do not increase the total amount of PGM compared to standard oxidation catalysts or even use less PGM to achieve better conversion of CO and HC according to the overall regulated emission test criteria. Will have an advantage. For this purpose, a "standard" oxidation catalyst has a uniform washcoat loading and a uniform PGM loading along its entire length.

In order to develop a novel oxidation catalyst, we have placed, in some motor vehicles, a relatively high amount of platinum in the zone at the inlet end of the catalyst substrate, such that this arrangement promotes faster light-off of the catalyst. It has been found that it can improve the conversion of CO and HC during the acceleration phase in ECE criteria. However, it has been found that during the cooling phase of the ECE cycle, such as deceleration, the front side of the catalyst is light-out.

For the purposes herein, "activation" is defined as the temperature at or above which the catalyst catalyzes the conversion of the reactants to an efficiency of at least 50% or T50. "Deactivation" means that the catalyst temperature has dropped below the level that catalyzes the conversion of the reactants with an efficiency of at least 50%. It is also possible to select another measure of determination of activation, such as T70, i.e., the temperature at which the catalyst catalyzes the conversion of the reactants at an efficiency of at least 70% or higher, but for purposes herein, " activation " Will mean.

We have developed oxidation catalysts for lean combustion engines that do not use more PGMs than standard oxidation catalysts and actually use less PGMs to achieve better conversion of CO and HC according to regulated emission test criteria than standard oxidation catalysts. .

According to one aspect, the present invention

One or more platinum group metals (PGMs) supported on the surface area-increasing washcoat component, having a substantially uniform length, measured from the upstream end and the inlet end at the inlet end of the flow-through substrate monolith. A first washcoat zone defined by a downstream end at a point less than halfway along the length of the substrate monolith;

A second washcoat zone containing at least one PGM supported on the surface area-increasing washcoat component, having a substantially uniform length and comprising an intermediate point along the length of the substrate monolith measured from the inlet end; And

Containing at least one PGM supported on the surface area-increasing washcoat component, having a substantially uniform length, and a maximum of three quarter points along the length of the substrate monolith measured from the downstream and inlet ends at the outlet end of the substrate monolith; A third washcoat zone defined by an upstream end in

Including, at this time

PGM loading in the first washcoat zone and PGM loading in the third zone is greater than PGM loading in the second zone,

Wherein the first washcoat zone comprises a washcoat load of less than the washcoat load of the third washcoat zone,

An exhaust system for a lean burn internal combustion engine is provided that includes a catalyst for oxidation of carbon monoxide (CO) and hydrocarbon (HC) comprising a distribution substrate monolith.

Without wishing to be bound by theory, the inventors have found that the present invention can achieve better CO and HC conversion for NEDC test criteria than standard oxidation catalysts or even two-zone oxidation catalysts consisting of more PGM loading zones at the inlet end. It is believed that this is because the third washcoat zone will maintain activation during the cooling phase of the cycle, while the standard or two-zone oxidation catalyst will be “inactive”. The second washcoat zone can act as a heat buffer and can provide HC and CO conversion under high speed conditions in the EUDC portion of the NEDC reference. Also in embodiments where the second washcoat zone comprises a suitable washcoat component such as zeolite, the second washcoat zone can provide HC storage during the colder stages of the cycle and subsequent release and conversion during the hotter stages. have.

In certain embodiments, the catalyst is substantially free of alkaline earth metals and alkali metals, i.e., the catalyst intermittently concentrates the exhaust gas, for example to adjust the air to fuel ratio in the engine, or to open the engine cylinder before opening the exhaust valve. NO _x absorbents in the exhaust system which desorb the absorbed NO _x and convert it to N ₂ using a suitable reducing agent by pre-injecting or post-injecting the fuel or by directly injecting the reducing agent downstream of the exhaust gas of the exhaust manifold It is not intended for use as (such as disclosed in WO 01/74476). In another embodiment, the catalyst is substantially free of lanthanide metals such as cerium.

In one embodiment, the downstream end of the first washcoat zone is 15-70% along the length of the substrate monolith from the inlet end, and the second washcoat zone comprises 10-80% of the total substrate monolith length. In another embodiment, the downstream end of the first washcoat zone is 10-65% along the length of the substrate monolith from the inlet end, and the second washcoat zone comprises 10-80% of the total monolith length.

In another embodiment, the upstream end of the third washcoat zone is 70-15% along the length of the substrate monolith from the inlet end and the second washcoat zone comprises 10-80% along the total substrate monolith length. In another embodiment, the upstream end of the third washcoat zone is 65-10% along the length of the substrate monolith from the inlet end, and the second washcoat zone comprises 10-80% of the total substrate monolith length.

In one embodiment, the total PGM loading in the first washcoat zone is 10 gft ^-3 to 240 gft ^-3, In another embodiment, the total PGM loading in the first washcoat zone is 30 gft ^-3 to 240 gft ^{- 3}

In another embodiment, the total PGM loading in the third washcoat zone is 10 gft ^-3 to 120 gft ^-3 , and in certain embodiments, the PGM loading in the first washcoat zone is equal to the PGM loading in the third washcoat zone. same. In another embodiment, the total PGM loading in the third washcoat zone is 60 gft ^-3 to 120 gft ^-3 .

The total PGM loading in the second washcoat zone can be 5 gft ^-3 to 30 gft ^-3 .

The total PGM loading on the entire substrate monolith can be 25 gft ^-3 to 390 gft ^-3 , such as 30 gft ^-3 to 240 gft ^-3 or 50 gft ^-3 to 120 gft ^-3 . "PGM" herein includes platinum, palladium, rhodium and iridium. In an exemplary embodiment herein, the PGM of the first, second and third washcoat zones consists of platinum.

Typically, the surface area-increasing washcoat component is an oxide selected from the group consisting of zeolites, alumina, titania, silica, ceria, zirconia and mixtures thereof and mixed and complex oxides containing any two or more thereof. In certain embodiments, the first washcoat zone includes a washcoat having a lower heat capacity than the washcoat of the second or third washcoat zones.

In this regard, the washcoat loading in the first washcoat zone may be 0.5 gin ⁻³ to 2.5 gin ⁻³ , and the washcoat loading in the third washcoat zone may be 1.5 gin ⁻³ to 5.0 gin ⁻³ . In one embodiment, the washcoat loading in the second washcoat zone is equal to the washcoat loading in the third washcoat zone.

In certain embodiments, the washcoat loading in the third washcoat zone is at least 10% greater, eg, at least 15% or at least 20% greater than the washcoat loading in the first or second washcoat zone. Alternatively, the washcoat loading in the second and third washcoat zones is at least 10% greater than, for example, at least 15% or at least 20% greater than the washcoat loading in the first washcoat zone.

Of course, in the first step, after coating the substrate monolith with a first washcoat zone having a lower heat capacity than the second washcoat zone and a second washcoat zone contiguous with the first washcoat zone, then on the coated substrate monolith PGM-containing zones may overlap. That is, two of the PGM zones need not be defined as washcoat zones, for example, the second washcoat zone may start at a portion of the second PGM zone.

The advantage of this arrangement is that higher heat capacity washcoats in the third washcoat zone or in the second and third washcoat zones provide longer heat retention during the cooling phase of the NEDC, ie the oxidation catalyst is provided on the back of the substrate monolith. To retain activation as much as possible by having a higher thermal mass of thermal energy; Lower frontal washcoat loading for this provides faster activation during the heating phase of the test. By placing the PGM in at least three zones along the length of the substrate monolith and increasing the thermal mass only in the second and third washcoat zones, or the third washcoat zone, the overall CO and HC conversion in the NEDC will be much improved. Can be.

One way of obtaining higher heat capacity in the downstream washcoat zone is to use thicker washcoats, as described in US Pat. No. 6,827,909 Bl. However, thicker washcoats can increase back pressure with systems outside the desired design tolerances and can even cause barrier formation on distribution monolithic substrates, especially on higher cell density monoliths, during manufacturing. However, in certain embodiments, the present invention uses washcoat components that have inherently higher heat capacities to reduce or overcome the problems associated with thicker washcoats. Examples of higher heat capacity washcoat components include densified alpha alumina, densified zirconia, densified lantana and densified ceria.

Typical properties for such materials are low surface area, fire resistance and glassy consistency. Such materials are also known to be "fused". For example, the density of fused zirconia is 5.90 gcm ^-3 ; Densified alpha alumina is 3.97 gcm ^-3 ; Densified Lantana is 6.5 gcm ^-3 ; Densified ceria (cerium II oxide) is 7.1 gcm ^-3 . Any material having a density of at least 3.5 gcm ^-3 applies to this aspect of the invention.

The preparation of the oxidation catalyst according to the invention can be achieved using methods and devices known in the art, including those disclosed in WO 99/47260, and therefore are not described in more detail here.

According to another aspect, the present invention provides a device comprising a diesel engine such as a light load diesel engine and an exhaust system according to the invention. In certain embodiments, the catalyst for oxidizing CO and HC may be referred to as a diesel oxidation catalyst or DOC.

In order that the present invention may be understood in more detail, embodiments and examples are provided and accompanying drawings are referred to for reference, but these are merely exemplary.

1 is a schematic diagram showing a cross-sectional view of a substrate monolith comprising an embodiment of an oxidation catalyst used in the present invention.

2 is a graph showing the cumulative CO emissions of a vehicle equipped with an exhaust system according to the present invention running on NEDC basis compared to a standard exhaust system.

3 shows 1) a standard oxidation catalyst, 2) a three PGM zone oxidation catalyst according to the invention, 3) a two zone washcoat oxidation catalyst (for comparison only) and 4) three PGM zones according to the invention and Bar chart showing average exhaust pipe emissions including HC, CO and NO _x emissions for NEDC of a vehicle equipped with a combination of two zone washcoat oxidation catalysts.

In one aspect, FIG. 1 shows a longitudinal section of a distribution substrate monolith 10, such as a conventional cordierite 400 cell / inch ² (cpsi (62 cell cm ⁻² )) structure. The substrate monolith 10 is also used as a directional arrow in a plan view showing the exhaust gas flow direction of the substrate monolith (12) with platinum carried on the alumina washcoat 14 in the zone defined at the downstream end 16. Coated). The second washcoat zone 18 is defined as its upstream end at the downstream end 16 of the first washcoat zone and downstream end at the upstream end 20 of the third washcoat zone 22. The second washcoat zone 18 includes platinum supported on the alumina washcoat at a lower platinum loading than the first washcoat zone 14. The third washcoat zone 22 is defined as its downstream end at the outlet end 24 of the substrate monolith 10 and comprises platinum supported on the alumina washcoat in a similar platinum loading as the first washcoat zone. . The first washcoat zone is 30% of the total length of the substrate monolith and the third washcoat zone is 20% of the total length of the substrate monolith. The washcoat loading in the second and third platinum zones may be greater than the washcoat loading in the first platinum zones.

Example  One

The 1.5 l capacity TDI passenger car according to the Euro III passenger emission standard was equipped with a substrate monolithic coated with a standard homogenous oxidation catalyst washcoat loaded with platinum along its total length. The vehicle was then tested according to the NEDC test criteria. At the same time, the same vehicle was equipped with a substrate monolith comprising a 3-zone Pt oxidation catalyst / 2-zone washcoat supported oxidation catalyst according to the present invention, wherein the total Pt loading was the same as the standard oxidation catalyst, and the NEDC test Was repeated. The cumulative carbon monoxide conversion results (left axis) superimposed on the velocity plot (right axis) of the NEDC test criteria are shown in FIG. 2. The top CO graph shows engine-out CO, which was the same in both tests. It can be clearly seen from the lower graph that the CO accumulated in the oxidation catalyst according to the invention achieved about 30% better CO conversion for NEDC than the standard oxidation catalyst.

Example  2

3 shows the results of a series of NEDC tests performed on a 1.3 l capacity TDI passenger car according to Euro III emission criteria. In the bar chart, 1) is a standard oxidation catalyst, 2) is a three PGM zone oxidation catalyst according to the present invention, 3) is a two zone washcoat oxidation catalyst according to the present invention, and 4) is according to the present invention. It is a combination of three PGM zones and two zone washcoat oxidation catalysts. The amount of Pt supported was 70 gft ^{- 3} in 1) -4).

It can be seen from FIG. 3 that both the three Pt zone oxidation catalysts and the two washcoat zone oxidation catalysts were improved over the standard homogeneous oxidation catalyst, and the total Pt was used less. It can also be seen that the combination of three PGM zones and two zone washcoat oxidation catalysts according to the present invention is further improved compared to three Pt zone oxidation catalysts and two zone washcoat oxidation catalysts. The conversion of HC, CO and NO _x for the combination of three PGM zones and two zone washcoat oxidation catalysts according to the invention compared to a standard oxidation catalyst is shown in Table 1.

Claims (19)

An upstream end containing at least one platinum group metal (PGM) supported on the surface area-increasing washcoat component, having a substantially uniform length, at the inlet end 12 of the flow-through substrate monolith 10. And a first washcoat zone 14 defined as a downstream end 16 at a point below midpoint along the length of the substrate monolith measured from the inlet end; A second washcoat zone 18 containing at least one PGM supported on the surface area-increasing washcoat component, having a substantially uniform length and comprising an intermediate point along the length of the substrate monolith measured from the inlet end; And Containing at least one PGM supported on the surface area-increasing washcoat component, having a substantially uniform length, and a maximum of three quarter points along the length of the substrate monolith measured from the downstream and inlet ends at the outlet end of the substrate monolith; Washcoat zone (22) defined as an upstream end (20) in Including, at this time PGM loading in the first washcoat zone and PGM loading in the third washcoat zone is greater than PGM loading in the second washcoat zone, Wherein the first washcoat zone comprises a washcoat load of less than the washcoat load of the third washcoat zone, An exhaust system for a lean burn internal combustion engine comprising a catalyst for oxidizing carbon monoxide (CO) and hydrocarbon (HC) comprising a distribution substrate unitary body (10). The exhaust system of claim 1, wherein the downstream end of the first washcoat zone is 15-70% along the substrate monolith length measured from the inlet end and the second washcoat zone is 10-80% of the total monolith length. The upstream end of the third washcoat zone is 70-15% along the length of the substrate monolith measured from the inlet end, and the second washcoat zone is 10-80% of the total monolith length. Exhaust system. The exhaust system according to claim 1, wherein at least one PGM is selected from the group consisting of platinum, palladium, rhodium, iridium and any two or more thereof. The exhaust system according to claim 1, wherein the PGM of the first, second and third washcoat zones consists of platinum. 6. The exhaust system according to claim 1, wherein the total PGM loading in the first washcoat zone is 10 gft ⁻³ to 240 gft ⁻³ . ⁷ . The exhaust system according to claim 1, wherein the total PGM loading in the third washcoat zone is between 10 gft ⁻³ and 120 gft ⁻³ . 8. An exhaust system according to claim 6, wherein the amount of PGM loading in the first washcoat zone is the same as the amount of PGM loading in the third washcoat zone. The exhaust system according to claim 1, wherein the total PGM loading in the second washcoat zone is between 5 gft ⁻³ and 30 gft ⁻³ . 10. The exhaust system according to claim 1, wherein the total PGM loading on the entire substrate monolith is 25 gft ⁻³ to 390 gft ⁻³ . The washcoat component according to claim 1, wherein the washcoat component contains an oxide selected from the group consisting of zeolite, alumina, titania, silica, ceria, zirconia and mixtures thereof and any two or more thereof. Exhaust system comprising mixed and complex oxides. The exhaust system according to claim 1, wherein the washcoat loading in the second washcoat zone is equal to the washcoat loading in the third zone. The exhaust system according to claim 1, wherein the washcoat loading in the first washcoat zone is between 0.5 gin ⁻³ and 2.5 gin ⁻³ . The exhaust system according to claim 1, wherein the washcoat loading in the third washcoat zone is between 1.5 gin ⁻³ and 5.0 gin ⁻³ . The exhaust system according to claim 1, wherein the heat capacity of the washcoat component in the third washcoat zone is originally greater than the heat capacity of the washcoat component in the first washcoat zone. The exhaust system of claim 15 wherein the washcoat loading in the first washcoat zone is substantially the same as the washcoat loading in the third zone. The exhaust system of claim 16 wherein the density of the washcoat component in the third washcoat zone is at least 3.5 gcm ⁻³ . 18. The exhaust of claim 16 or 17, wherein the washcoat component of the third washcoat zone is selected from the group consisting of densified alpha alumina, densified zirconia, densified lantana, densified ceria, and mixtures of any two or more thereof. system. An apparatus comprising an exhaust system and a diesel engine according to any of claims 1 to 18.

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